8 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
prominences are volcanic, as the summit of Mount Tapochau itself, of Marpi to the 
north, and of similar uplands. 
Tinian is low and flat. It is composed almost entirely of reef limestone, low 
on the western side and sloping upwards to about 150 feet on the east. A few 
hills of volcanic nature rise to various heights above the general limestone levels; 
the highest of these stands well to the southeast and attains an altitude of 300 feet. 
This island, in contrast with Saipan, closely resembles the northern half of Guam. 
Rota consists of two portions joined by an isthmus, both of which display the 
terraces of uplifted reefs like those of northern Guam and of Tinian, but with 
much greater distinctness. A volcanic nucleus exists in the larger element of 
Rota, and it rises to the somewhat remarkable height of 800 feet. 
THE SURROUNDING FLOOR OF THE OCEAN. 
Although the present researches are biological in character, they are neces- 
sarily concerned with the strictly geological problem as to the prior existence of a 
great land-mass, or a group of land-masses, in the western and southern Pacific 
Oceans. According to one hypothesis, the subsidence of such territory or territories 
would leave only the peaks above the water’s surface as the several islands now in 
existence. The opposed view is that volcanic upheaval has been the principal if 
not the sole process by which such small land areas have been formed. The 
pertinent facts of zoogeography are that the neighboring islands bear species of 
Partule that are closely similar or even specifically identical, as in the case of 
P. gibba, which exists in Guam, Tinian, and Saipan in the Mariana Group. On 
the basis of the subsidence hypothesis a widespread parental stock of the original 
land-mass would be separated into the isolated daughter stocks of disconnected 
islands, whereupon evolutionary divergence might or might not ensue. If the 
alternative hypothesis of uplift were true, then a newly arisen island must have 
gained its vegetation and its Partula population by immigration from other and _ 
older islands. Almost inevitably such immigrants would come from the nearer 
inhabited areas; as in the alternative case, their subsequent history might be one of 
diversification or it might not, as circumstances would direct. 
In the volume on the species of Tahiti, the position was taken that subsidence 
has occurred in the Society Islands, and that widespread ancestral stocks have been 
dissected, as it were, into the separate insular associations of to-day. It does not 
follow, of course, that every island group in Oceania has had the same history, or 
that the reverse phenomenon of uplift can not be manifested in an area in which 
subsidence is the dominant process; it is clearly apparent that much of the rock in 
the southern Mariana Islands, as in many another part of Oceania, was constructed 
as coral reef, and that the earth-strains of subsequent periods forced it up to high 
levels above the sea. In the view of the author at the present time, subsidence has 
been the general process by which an ancient land-mass has been converted 
into lesser disconnected areas, which in turn have been so depressed as to leave 
only the peaks above the sea, as the islands now existing in the Pacific. The 
history is a difficult one to establish and many geological considerations may be 
